My brother is about the same age as your stepson and is also trying to bulk. My brother isn't huge, but he managed to put on some mass. My brother and I are both naturally skinny and we found that the solution is simple. Eat more. I have to eat at least 5,000 calories a day to gain. I've read about men that have to eat far more than I do. Count his calories and keep adding more until he starts gaining.

Don't get him lifting too much until he finishes most of his growth spurts. I have friends (all over 6 feet) who had their little brother lifting weights by age 10 and he was enormous by age 12. He also ended up being the shortest male in his extended family by six inches.

I know it's been said already but the key is to work out consistently and eat. When I say eat, I don't mean eat when he's hungry, I mean eat like it's his job. He is growing and very active which requires lots of calories, he has to figure out how to have a calorie surplus despite his growth and activity level. This will likely require him to feel like he's over eating.

Starting Strength would be a great program to start with. If he starts now and doesn't see results in the coming months its because he isn't eating enough.